Field of the Invention
The invention is directed to a Bragg modulator that modulates, the intensity of a light beam, in particular of a laser beam. The Bragg modulator includes electro-optic crystal wafer, rectangular on its top face, in particular a lithium niobate or lithium tantalate crystal wafer. The wafer has a grid structure with normal zones and inversion zones. In the inversion zones, the direction of the spontaneous polarization is inverted compared to a normal direction in the normal zones. Electrodes are disposed on the top face and bottom face of the crystal wafer to generate an electric field therebetween.
It is known to use acousto-optic Bragg modulators for modulation of the intensity of laser beams. Such a Bragg modulator include an acousto-optic crystal, e.g., PbMoO.sub.4 or TeO.sub.2, and a convertor. Using the convertor, acoustic waves with a frequency of 200 to 300 MHz are excited in the acousto-optic crystal. The laser beam passing through the acousto-optic crystal is refracted by the acoustic waves, whereby with an adequately high acoustic frequency, it is possible to obtain only a refraction maximum. In this case, the angle of incidence of the optical wave on the acoustic wavefront equals the Bragg angle .theta..sub.B. This is governed by the equation: EQU sin .theta..sub.B -0.5.lambda..sub.o /.LAMBDA..sub.a n
for an optic wavelength .lambda..sub.o, an acoustic wavelength .LAMBDA..sub.a, and a relevant crystal refractive index n. The refracted light intensity is proportional to the acoustic intensity and can be controlled with the RF power at the convertor.
The know acousto-optic Bragg modulator has, however, several disadvantages. It is difficult to obtain rapid modulation of intensity, i.e., in the nano second range, since speed is limited by the propagation velocity of the acoustic waves. The reaction time of the modulator is determined by the length of time which the acoustic wave requires to pass through the laser beam. Consequently, fast switching rates require sharp focusing of the laser beam to obtain the smallest possible path for the acoustic waves. The acoustic waves typically have a velocity V.sub.a of approximately 4000 m/s in acousto-optic crystals. For example, for a switching rate of t.sub.s =10 ns, the laser beam must be focused on a spot with a diameter D=V.sub.a.multidot.t.sub.s =40 .mu.m. Such a requirement makes the entire system very sensitive mechanically, and special stabilization measures are needed. Limits are also imposed on focusing the laser beam insofar as an admissible light intensity must not be exceeded, since such a high energy density can result in the destruction of the crystal.
In addition, such an acousto-optic Bragg modulator requires comparatively complicated control electronics to excite acoustic waves with a frequency of 200 to 300 MHz, and also to switch this high frequency on and off very quickly.
Moreover, the production technology of the acousto-optic Bragg modulator is not planar and is thus comparatively complex and expensive. Thus, a convertor designed as a thin crystalline lithium niobate wafer must be manufactured separately and subsequently glued onto the acousto-optic crystal, which itself is comparatively expensive.
Finally, it must be noted that a high RF control power of 1 to 2 W is essential for the acousto-optic Bragg modulator. The high power may result in nonhomogeneous heating of the crystal, which, causes various instabilities.
Also known is a Bragg modulator that modulates the intensity of a light beam. This modulator includes of an electro-optic crystal wafer with a rectangular top face, and a grid structure with normal zones and inversion zones. In in the inversion zones, the direction of the spontaneous polarization is inverted compared to a normal direction in the normal zones. Electrodes are provided on the top face and bottom face of the crystal wafer. Such a Bragg modulator is shown in WO 96/07949 A1. In this Bragg modulator, the grid structure runs from the front face of the crystal wafer, into which the light beam is coupled, to the rear face, from which the unrefracted light beam passing through the grid structure exits the crystal wafer. The light beam refracted on the grid structure here strikes a side face of the crystal wafer.
Disadvantageous in this type of Bragg modulator include that the light beam exiting the side face is relatively useless, since the light beam striking a relatively small angle on the side face suffers high reflection losses (up to the total internal reflection). Because of overlapping with the unrefracted light beam, the light beam which is mostly (or entirely) reflected on this side face can moreover disadvantageously result in unwanted interference and noise.